Process of making fibers

ABSTRACT

An improved process of manufacturing fibers by the technique of forming a mixture of polymer, solvent for such polymer and, optionally, water or other flashing aids, at a temperature (flash temperature) which is high enough to bring the polymer to a plastic state and which will permit substantially complete vaporization of the solvent when the mixture is flashed, flashing the mixture into a flash zone to produce a fibrous product, and subsequently refining the fibrous product characterized in that the fibrous product is first passed through a primary refining zone at a temperature above the boiling point of the solvent, and then subjecting the fibrous product to a secondary refining. The primary refining is effected under conditions such that only a light defibering action is imparted to the fibrous product, and is preferably carried out in a disc refiner having wide plate clearances. The secondary refining is carried out under conditions to impart a vigorous fiberillating action to the fibrous product and provide a pulp having a drainage factor in excess of 1.0 second/gram.

United States Patent 1 Kozlowski [4 1 Sept. 2, 1975 PROCESS OF MAKING FIBERS [75] Inventor: John H. Kozlowski, Vancouver,

Wash.

22 Filed: Apr. 5, 1973 21 Appl. No.: 348,352

3,382,140 5/1968 Henderson ct al.... 162/100 3,436,304 4/1969 Spence 162/157 R 3,445,329 5/1969 West ct al....... 162/28 3,743,272 7/1973 Nowotny et a1. 162/157 R 3,743,570 7/1973 Yang ct a1 162/157 R 3,770,856 ll/l973 Veki ct al. 264/13 FOREIGN PATENTS OR APPLICATIONS 1,043,762 9/1966 United Kingdom 264/D1G. 8

Primary ExaminerS. Leon Bashore Assistant ExaminerPeter Chin Attorney, Agent, or Firm-Corwin R. Horton; Stanley M. Teigland; Robert E. Howard [5 7] ABSTRACT An improved process of manufacturing fibers by the technique of forming a mixture of polymer, solvent for such polymer and, optionally, water or other flashing aids, at a temperature (flash temperature) which is high enough to bring the polymer to a plastic state and which will permit substantially complete vaporization of the solvent when the mixture is flashed, flashing the mixture into a flash zone to produce a fibrous product, and subsequently refining the fibrous product characterized in that the fibrous product is first passed through a primary refining zone at a temperature above the boiling point of the solvent, and then subjecting the fibrous product to a secondary refining. The primary refining is effected under conditions such that only a light defibering action is imparted to the fibrous product, and is preferably carried out in a disc refiner having wide plate clearances. The secondary refining is carried out under conditions to impart a vigorous fiberillating action to the fibrous product and provide a pulp having a drainage factor in excess of 1.0 second/gram.

16 Claims, 2 Drawing Figures PROCESS OF MAKING FIBERS BACKGROUND OF THE INVENTION Numerous processes have been proposed for preparing synthetic fibrous materials by flashing polymer solutions or dispersions held at high temperature and pressure into a zone of reduced pressure. In various patent literature, such as German Offenlegungsschrift No. 1,958,609 and Japanese patent application having publication No. 71-34921, processes are proposed in which a polymer is dissolved in a solvent therefor and heated under at least autogenous pressure and then flashed into a zone of lower pressure to thereby vaporize the solvent and form fibrous materials. In the latter mentioned patent, the fibrous material thus formed is quenched with a water spray at a temperature between 60 C. and 80 C.

Similar processes are presented in United States patent application Ser. NO. 295,339, filed Oct. 5, 1972, (assigned to the assignee of the present application) and as well in German OLS NO. 2,121,512 and German OLS No. 2,144,409. In theseprocesses a polymer dissolved in a solvent is mixed with water or other liquid nonsolvents for the polymer to form an emulsion of the polymer solution in a continuous water phase and this emulsion is heated and flashed to a reduced pressure zone to produce fibers.

Another approach is described in US. patent application Ser. No. 285,386, filed Aug. 30, 1972 now abandoned, (assigned to the assignee of the present application) wherein a polymer solution in which water is dispersed as a discontinuous phase is flashed to form fibers. In this process, the water concentration is held between 30% and 70% of the entire mixture and in forming the mixture it is preferable to add the water to the preformed polymer solution to insure that the water forms the discontinuous phase.

OLS No. 2,147,461 describes a process in which molten polymer is emulsified with water (optionally, with a minor amount of solvent in the polymer phase) and then heated and flashed to a reduced pressure zone to form fibers.

While many variations are evident, it is seen that the common feature of all these fiber-making processes is the flashing into a zone of reduced pressure of a heated mixture containing a polymer and a solvent for such polymer. Various conditions of flashing are suggested in the referenced processes, including temperature and pressure ranges for the mixture to be flashed and various solvents, flashing aids, etc. Most of the references contemplate flashing into a zone held at atmospheric pressure. Others suggest the possibility of flashing into a zone which is above or below atmospheric pressure, in some cases with the application of heat in such zone. All of these processes may lead to the production of fibrous material. However, each suffers from the shortcoming that a specific set of conditions of flashing, particularly with regard to temperature and pressure, and interrelationship of such conditions are not provided which will permit manufacture, in a practical manner, of fibers having the optimum properties desirable for their use as a synthetic pulp in the manufacture of paper by conventional techniques.

Fibrous materials produced under the general process conditions described in these references tend to be interconnected or bundled together toan undesirable degree and the paper produced therefrom isundesirably low in strength (e.g., tensile strength). Such fibrous material is more difficult to separate, cut or refine in preparation for papermaking and contains a high content of gels and chunks of polymer which cause undesirable fish eyes or transparent spots in paper manufactured therefrom.

It has recently been suggested (in US. patent application Ser. NO. 340,140 of H. Yonemori, filed by the present assignee on Mar. 12, 1973, and entitled Pro-.

cessof Making Fibers) to overcome these problems by providing a specific interrelated set of flashing conditions to produce a product capable of being readily separated and refined into an improved pulp for papermaking which has a low content of gels and chunks and excellent drainage factor characteristics. Briefly, that process comprises flashing into a zone at subatmospheric pressure to provide a fibrous product having a temperature less than C. and subsequently refining the pulp at this low temperature, preferably while maintaining the pulp under such subatmospheric conditions in order to minimize condensation of solvent vapor. While this approach is satisfactory from a technical point of view, it is expensive to practice commercially because of the vacuum requirements imposed on the system. Also, the amount of unvaporized solvent remaining in the fibrous product after refining is such as to require further solvent elimination measures.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The purpose of the present invention is to provide an economical technique applicable to all of the flashing processes previously described which will produce a pulp useful for producing synthetic paper by conventional papermaking techniques which has surprisingly improved properties and a low content of solvent, gels and chunks.

Briefly, the present process comprises taking the flashed fibrous product produced by the foregoing processes, forming an aqueous mixture of the fibrous product while maintaining the product at a temperature above the boiling point of the solvent(s) employed in the solution or dispersion, passing the fibrous product at such a temperature through a primary refining zone operated under conditions such that the fibrous product is subjected only to a light shredding or defibering action, and passing the fibrous product through a sec ondary refining zone under conditions such that the fibrous product is given vigorous fibrillating action. Preferably, the fibrous product is cooled prior to secondary refining.

DESCRIPTION OF PREFERRED EMBODIMENTS ln practicing the process of the present invention, any polymer or copolymer may be employed which is capable of forming fibers by conventional spinning techniques. It is preferred to employ crystalline or partially crystalline polyolefins such aslow pressure polyethylene, isotactic or partially isotactic polypropylene, and ethylene-propylene copolymers. Additionally, polybutenes and polymethyl pentenes may be employed in the practice of this invention. Crystalline or partially crystalline polyamides and polyesters may also be used. Noncrystalline polymers such as polycarbonates, polysulfones, polyvinyl chloride, polymethylmethacrylate, polyacrylonitrile and polystyrene may be used. Mixtures of the foregoing with each other or other polymers may also be employed.

The preferred polyolefins employed are those having an intrinsic viscosity above about 0.7 dl/g., which for high density polyethylene corresponds to a viscosity average molecular weight of about 30,000 to 40,000.

The polymers employed in practicing the present process may be in the form of dried powder or pellets or, preferably, as a wet cake, slurry or solution of polyolefin in the reaction solvent as obtained after polymerization.

Generally, any substituted or unsubstituted aliphatic, aromatic or cyclic hydrocarbon which is a solvent for the polymer at elevated temperatures and pressures, which is relatively inert under the conditions of operation and which has a boiling point at atmospheric pressure that is between 20 C. and 100 C., preferably between 50 C. and 100 C., and at the flash zone temperature less than the softening p3 int of the polymer may be employed in practicing the present process. Illustrative of the solvents which may be utilized are aromatic hydrocarbons, e.g., pentane, hexane, heptane and their isomers and homologues; alicyclic hydrocarbons, e.g., cyclohexane; chlorinated hydrocarbons, e.g., methylene chloride, carbon tetrachloride and chloroform; higher alcohols; esters; ethers, ketones; nitriles, amides; fluorinated compounds, e.g., fluoro-hydrocarbons; nitromethane; and mixtures of the above solvents and other solvents having a boiling point between about 20 C. and 100 C. at atmospheric pressure. The preferred solvent is hexane which has a boiling point of 68.7 C. at atmospheric pressure.

The polymer-solvent mixture may be formed by any one of several methods. One may start with a solution of polymer in solvent as it comes from a solution polymerization process, either at the same concentration, diluted or concentrated. Alternatively, one may start with a slurry of polymer particles in the solvent such as is produced by a slurry of polymerization procedure and the appropriate amount of water is added to the slurry or vice versa. A further alternative would be to start with a dry polymer powder, or granules, or a wet cake such as might be produced at some stage of solvent removal in the polymer plant and the appropriate amount of solvent is admixed therewith.

The polymer concentration relative to the solvent is not critical, the solvent being present in an amount that is greater than 100% by weight of the polymer and sufficient to give a viscosity at the flash temperature employed that can be easily handled. Frequently, this viscosity will be up to about 3,500 centipoises. Generally, the polymer concentration will vary from about 2% to about 30% by weight of the solvent plus polymer, and preferably is in the range of about 5% to about In one preferred embodiment water is employed as a flashing aid. In this embodiment the water may be in a continuous phase or discontinuous phase, depending upon the amount of water added to the polymersolvent mixture and the manner of addition. If the water is to form the discontinuous phase it should be present in an amount less than 70%. To form a continuous water phase the water should be present in an amount greater than 30% by volume of the mixture and preferably between 50% and 70%. The particular method of mixing is not critical, but if it is desired to have the water form a discontinuous phase it has been found to be advantageous to have the solvent present prior to water addition since the solvent or polymer solution will form the continuous phase of the mixture to be formed. This latter approach is particularly desirable when one employs an amount of water which is near the borderline of an inversion occurring, i.e., at the point where the amount of water is approaching that level where it would form the continuous phase. Conversely, if the water is added with or before the solvent, it will tend to form the continuous phase.

A primary function of the water is to provide energy to aid the vaporization of the solvent during flashing since it is not desirable to have the temperature so high that there is sufficient energy imparted to the solvent alone to effect its complete vaporization. However, the amount of water should not be so great as to require the expenditure of unnecessary heat values in attaining the desired flashing temperature, i.e., once that amount of water required to form an aqueous solution or dispersion of the agent having a suitable viscosity is determined, additional water may be employed to a certain extent since it helps to lower the mixture viscosity and aids solvent vaporization but the additional amount need not be great.

Another function of the water is to reduce the temperature of the fibrous mass in the zone immediately following the nozzle (flash zone). The addition of water increases the total vapor pressure of the system at the moment of flashing thus reducing the boiling point of the flashing mixture. This is independent of the amount of water employed, and very small quantities may thus be employed for this purpose. As a practical matter, however, water would be employed in the amount of at least about 1% by volume of the solvent-water mixture. Lowering the boiling point of the mixture in this fashion will assist in establishing proper temperature conditions in the fibrous mass formed upon flashing in accordance with this invention, as discussed in detail at a later point.

Another function of the water is to act as the carrier for a hydrophilic water-dispersing agent for the fibers to be formed. It has been found that it is most advantageous to have the water-dispersing agent present during flashing and precipitation of the fibrous polymer. An equivalent amount of the same agent added at a later stage to the already formed fibers does not give the same degree of dispersibility and the presence of the agent enhances the refinability of the fibers. Therefore, the water should be present in an amount sufficient to carry that amount of the hydrophilic agent employed to impart to the fibrous polymer the desired level of water dispersibility, preferably as a solution thereof. Additional water above such minimum amount required to carry the agent may be employed to impart a suitable viscosity to the aqueous solution or dispersion agent, i.e., the aqueous solution of the water-dispersion agent should not be so viscous as to present problems of han dling or incorporation into the polymer solution as a dispersed phase. Also, the water can aid in reducing the viscosity of the mixture to a level less than that of the polymer solution alone, thus permitting higher polymer concentrations.

The agents which may be added to the mixture to impart water dispersibility to the fibrous polymer are preferably Water soluble, or partially water soluble, high molecular weight materials. However, they may also be materials which are soluble or partially soluble in the solvent so long as they are somewhat hydrophilic and impart water dispersibility to the fibers. The amount of water-dispersing agent employed may range from about 0.1% to about by weight of the polymer, preferably from about 0.1% to about 5% by weight. The preferred waterdispersing agent is an at least partially water-soluble polyvinyl alcohol (PVA) having a degree of hydrolysis greater than about 77% and, preferably, greater than about 88 mo]. 7c and having a viscosity (in a 4% aqueous solution at C.) between about 2 and 50 centipoises. Desirably, the PVA has a degree of polymerization in the range of 200 to 4,000 and preferably between 300 and 1,500. If desired, the PVA may be chemically modified to enhance its adhesion to the polymer, dispersion and other properties. The polyvinyl alcohol is preferably added with the water at the time the mixture is formed. Illustrative of other water-dispersing agents that may be employed are cationic guar, cationic starch, potato starch, methylcellulose and Lytron 820 (a styrene-maleic acid copolymer).

Such water-dispersing agents are also advantageous in developing the fiber properties during refining of the flashed product in accordance with this invention, as described at a later point. Alternatively for this purpose, such agents may be added subsequent to flashing, such as with the dilution water for refining. It is particularly advantageous to add at least 0.75% and preferably between about 1 /z% and 5% by weight of the flashed polymer of polyvinyl alcohol, either before and/or after flashing (but prior to refining).

The ingredients of the mixture can be placed in any suitable vessel which is capable of being heated to an elevated temperature and pressure. Generally, an autoclave is employed. However, when water is added as a flashing aid it is important that the vessel employed be equipped with a mixing or stirring device capable of keeping the mixture in a constant state of agitation since a stable emulsion is not formed and upon standing the mixture will quickly separate into two distinct and separate phases.

The ingredients are then heated to a suitable temperature and preferably agitated if water is present to form a uniform mixture wherein Water is present as a discontinuous or dispersed phase within a continuous phase of polymer solution or as a continuous phase with polymer solution dispersed uniformly therein, depending upon the water concentration and mode of addition, as previously discussed. The temperature employed is preferably above the melt dissolution temperature of the polymer in the solvent employed. The melt dissolu tion temperature of any particular solvent is determined by placing low concentrations of the polymer (e.g., 0.1 and 1.0% by weight) into the solvent in a vial which is then sealed and placed in an oil bath. The temperature of the oil bath is raised slowly (e.g., 10 C/hr) until the last trace of polymer disappears. This temperature is the melt dissolution temperature. In some instances, it may be desirable to operate at a temperature below the melt dissolution temperature. In this case the temperature should be high enough under the operating conditions so that the polymer is dissolved in the solvent or at least is in a swollen state with sufficient fluidity to be discharged from the nozzle, i.e., in a plastic state.

Flashing is preferably carried out substantially adiabatically, utilizing the heat (enthalpy) in the heated mixture to provide the heat of vaporization for vaporizing substantially all of the solvent when the mixture is discharged to the flash zone held at a suitable lower pressure. Accordingly, for adiabatic flashing, the temperature of the mixture prior to flashing should be high enough to provide sufficient heat content or enthalpy for vaporization adiabatically of substantially all solvent upon flashing to the flash zone. However, the maximum temperature employed should be less than the critical temperature of the solvent and/or the decomposition temperature of the polymer.

It is also possible to carry out the flashing to some extent in a nonadiabatic fashion, for example, by the addition of heat to the material as it is flashed from the nozzle. For instance, low pressure steam (e.g., below 20 psig) or water at about 100 C. may be added to the fibrous noodle in the flashing zone as by injection thereof in a conduit immediately following the flash nozzle into which the flashed noodle is also injected. In this case, the flashing temperature should be chosen so the heat content in the mixture to be flashed plus the heat added to the flashed material is sufficient to vaporize substantially all of the solvent in the flash zone.

The pressure employed in the vessel containing the heated mixture is preferably substantially autogenous although pressures higher than autogenous may be employed. It may be desirable, particularly in batch operations, to employ an inert gas such as nitrogen during the flashing operation to maintain substantially autogenous pressure in the vessel and thus maintain the velocity of the mixture through the nozzle at a fairly constant level.

Flashing is preferably effected through a nozzle which has a substantial longitudinal dimension in order to efficiently impart shear to the mixture (particularly the polymer component thereof) immediately prior to flashing. Such shearing action aids fiber formation and enhances fiber properties for papermaking purposes. The nozzle may be circular or noncircular in crosssection and may be an annulus.

The flash zone is maintained at a pressure which, in conjunction with the other conditions of flashing are selected so that the temperature of the flashed product is almost immediately lowered in the flash zone, by evaporation of substantially all of the solvent (and a portion of the flashing aids, if employed), to a tempera ture below the softening temperature of the polymer, preferably below about 100 C., but above the boiling temperature of the solvent(s) employed and preferably above about C.

In a typical flashing procedure in accordance with this invention, vaporization may be substantially complete 10 to cm downstream of the nozzle. However, this can vary widely depending on the flow veloc ity, flash zone pressure, flash temperature, solvent, etc.

lmportantly, in the mixture to be flashed, all of the components of the mixture and the concentration of each in the mixture are chosen with respect to the heat capacity of each, with respect to the heat of vaporization of each component which will be volatilized in flashing, and with respect to the flash temperature chosen so as to produce in the flashed product a temperature above the boiling point of the solvent and preferably between about 70 C. and about 100 C. upon flashing of the mixture into the flash zone. Expressed in another way, the heat content of the mixture to be flashed and the heat to be removed through vaporization of the vaporizable components (solvent and any flashing aids vaporized, if employed) should be adjusted so that the residual heat in the flashed product, after removal of the heat of vaporization of the vaporized components, will impart a temperature in the flashed product which is between about 70 C. and 100 C. Selection of the appropriate flashing temperature and of the components of the mixture and their concentrations for this purpose will depend upon the pressure selected for the flash zone and the amount of heat added to the flash material during flashing if the flashing is performed nonadiabatically. Preferably, the flash zone is at substantially atmospheric pressure, i.e., between about 600 and 800 mm Hg. If the flash temperature is too high or if the components of the flash mixture and their concentrations with respect to their heat capacity and heat of vaporization (for the vaporizable components) are improperly chosen, the temperature of the flashed product, upon substantially complete evaporation of the solvent, will remain above 100 C. If the flash temperature is too low, or, again, the components are improperly chosen with respect to heat capacity and heat of vaporization, there will be incomplete evaporation of the solvent. For the purpose of this invention appropriate selection of these variable parameters, namely, the flash temperature, components and concentrations thereof in the flash mixture, may be determined for a given pressure condition in the flash zone by making a heat balance for the flashing operation which will produce the desired noodle temperature below 100 C. Advantageously, these variable parameters may be selected so as to satisfy the following equation:

Q I Enthalpy of polymer(s) in flash mixture Q, Enthalpy of solvent(s) in flash mixture Q,= Enthalpy of flashing aid(s) in flash mixture Q,,,, Heat added to flashing mixture (if nonadiabatic) V, Enthalpy of vaporization of solvent vaporized V Enthalpy of vaporization of flashing aids vaporized (if any) W,, Weight of polymer in the flashed product C',, Heat capacity of polymer in flashed product W} Weight respectively, of the flashing aids, adjuvants, and/or nonvolatile components other than polymer in the flashed product (if any) C; Heat capacity of the respective flashing aids, adjuvants, and/or nonvolatile components other than polymer in the flashed product (if any) and where such enthalpy values are based on the same temperature datum plane and the heat capacities are those applicable between such datum plane and the temperature of the flashed product.

In using this formula, the heat capacity values and Weights of the flash components may be substituted into this formula, for example, as follows for the specific case where a single polymer, solvent and flashing aid are present in the flash mixture:

a 70 c. WIT],

where the additional terms are:

W,- Weight of the solvent in the flash mixture C,- Heat capacity of the solvent in the flash mixture W Weight of the flashing aid in the flash mixture C Heat capacity of the flashing aid in the flash mixture C,, Heat capacity of the polymer in the flash mixture H Heat of vaporization of solvent under flash conditions H, Heat of vaporization of flashing aid under flash conditions T Flash temperature, C. and where the indicated heat capacities are those applicable between the flash temperatures and the previously mentioned temperature datum plane.

In practice, the heat capacity and enthalpy of vapori zation values for the desired components can be substituted into this equation. Then, a flash temperature and the concentrations of the flash mixture components may be selected relative to each other to satisfy the equation for a desired flashed product temperature. For ease of calculation it can be useful to program these variables for computer analysis to select the desired components and flash temperature.

It is not necessary to actually measure the noodle temperature (which is a cumbersome procedure under the flash zone conditions). All that is necessary for control purposes is to maintain the indicated parameters for the flash procedure at values that satisfy this equation for the noodle temperature which is desired. Of course, the various parameters should also satisfy the other conditions for proper flashing as previously discussed, e.g., the flash temperature should be above the melt dissolution temperature, substantially all of the solvent should be vaporized, etc.

While it is desirable to have the temperature of the fibrous product or noodle at a temperature between about C. and C. after flashing in order to insure vaporization of as much of the solvent as possible, the temperature can be raised to this range by suitable means prior to primary refining. As will be discussed more below, primary refining is carried out at a temperature above the boiling point of the solvent and preferably within the range of about 70 C. to 100 C. in order to vaporize any residual solvent left in the noodle.

During flashing, the polymer is precipitated as a fi' brous product or noodle," which is a loose aggregation of fibers which is usually continuous. The fibrous product is collected in a suitable receiving vessel, preferably one which permits the vaporized solvent to be separated therefrom.

The fibrous product in the receiving vessel may be either substantially dry if flashed from a polyolefin solution or as an aqueous slurry if flashed from a dispersion -or emulsion employing water as a flashing aid. However, in both cases a certain amount of unflashed residual solvent will remain with the fibrous product and it is therefore important that the temperature of the fibrous product be kept high enough to continue the vaporization of the solvent.

In the copending application of H. Vonemori referrcd to above, it was discovered that improved products were obtained by refining the fibrous product at relatively low temperatures, and to minimize condensation and/or retention of the sovlent, the vapor separation zone and the refining zone were desirably maintained under subatmospheric pressure.

The present invention resides in the discovery that similarly improved products can be obtained with a smaller amount of retained solvent by carrying out the refining of the fibrous product while it is maintained at an elevated temperature above the boiling point of the solvent if the fibrous product first passed through a primary refining zone under conditions such that only a light defibering or shredding action is imparted to the fibrous product. By defibering or shredding action, it is meant that the noodle is subjected to forces just sufficient to pull it apart into a fibrous mass which, at a consistency between about 1% and 10% by weight, is pumpable. If the fibrous product is subjected to more than such light defibering or shredding action while at elevated temperatures, the ultimate fiber and sheet properties are detrimentally affected, as will be shown in the examples.

Prior to passing the fibrous product through such primary refining zone, dilution water is added to the receiving vessel to provide an aqueous slurry of the fibrous product having an appropriate consistency. Typically, the consistency of the aqueous slurry is between about 1 to 10% by weight of the fibrous material, although high consistencies between about l0% and 60% by weight may be employed by using screw feeding or similar feeding means. The dilution water temperature should be such as to provide an aqueous slurry of noodle having a temperature above the boiling point of the solvent at the pressure employed (normally 1 atmosphere), and preferably between about 70C. and 100 C,

The aqueous slurry of fibrous product at an appropriate consistency and temperature is then passed through the primary refining zone to effect the desired defibering or shredding action on the material. In its preferred form, the primary refining zone is a disc refiner of the type conventionally employed in the papermaking art and may be a single or double rotating disc refiner. However, the refiner is operated to impart only a light defibering or shredding action to the fiberous product passing therethrough. This is accomplished by preferably employing plates having narrow bars, that is, bars having a width up to about 0.5 inch, and by spacing the plates relatively far apart, that is, a plate clearance distance between about 0.1 and 0.25 inch. The moving disc or discs are rotated at speeds to provide relative peripheral velocities between about 4,000 and 8,000 feet per minute. The size of the discs may be any of those normally employed in the papermaking art.

The defibering or shredding action referred to may be characterized by the amount of fibrous material rctained on the plus 35-mesh screens after primary refining when the fibrous material is classified in accordance with TAPPl Standard Test T-233 SU64. At least about 90% of the fibrous material should be retained on the 20 plus 35-mesh screens; if less than this amount is retained, the refining action has been too vigorous. Generally, the amount of work done on the fibrous material during primary refining will be less than about 0. l kilowatt-hour/kilogram.

While the primary refining is preferably accomplished by use of a disc refiner as discussed above, it may be accomplished in other devices such as Claflin refiners, Hollander beaters, Dynapulpers, hydrapulpers, etc., it only being necessary to impart a defibering or shredding action which will still permit at least about 90% of the fibrous material to be retained on a 20 plus 35-mesh screen.

While normally and preferably the amount of work to provide the light brushing action is imparted to the fibrous product by one pass through a disc refiner operated, as discussed previously, multiple passes may be employed as long as the total work imparted to the fibrous product by all the passes does not cause more than the defibering or shredding action just discussed.

The aqueous slurry of fibrous product resulting from the primary refining operation drops from the bottom of the disc refiner into a receiving vessel which may be purged with an inert gas such as nitrogen to remove solvent vaporized during the primary refining step. The fibrous product is preferably cooled to a temperature below C. by any suitable method, such as use of cooling coils, adding dilution water or sitting, and the cooled pulp is then subjected to secondary refining. Alternatively, the pulp may be cooled and thickened for storage and/or shipment, and the secondary refining may be accomplished at a much later date with the same beneficial results.

The secondary refining is preferably carried out in a disc refiner employing refiner conditions which impart a vigorous fibrillating action to the fibrous product which optimizes the fiber papermaking characteristics. The precise conditions employed in this secondary refining need not be gone into in detail since the skilled papermaker can optimize the type of plates, disc spacing, disc velocity and power output to develop optimum properties for the particular product to be produced from the fibers.

For purposes of illustration, the secondary refining may be carried out with refiner plates having a plate spacing less than 0.1 inch (2500 microns), and desirably between about zero and 0.01 inch (250 microns), and a disc peripheral velocity of between about 4,000 and 8,000 feet per minute. Multiple passes through one or more refiners may be employed. Instead of disc refiners, one may employ any conventional refining device and refine the pulp in a known manner to optimize properties.

The secondary refining is conducted under conditions such that the drainage factors of the resulting pulp is greater than l.O seconds/grams. Also, after secondary refining the pulp will be retained to the extent of less than and generally less than 50% by weight on the 20 plus 35-mesh screens.

By employing the refining sequence just described, i.e., a defibering or shredding-type primary refining of the fibrous noodle at an elevated temperature followed by secondary refining, preferably at a reduced temperature, a pulp is produced having superior qualities. In this respect, one indication of the improvement of the fibers made by the process of the present invention is the drainage factor of such fibers as compared to fibers of similar classified fiber length which are made using parameters outside the present invention. The drainage factor is a measure of the drainage characteristics of a fiber when a slurry thereof is placed on a foraminous surface. For synthetic fibers of similar fiber length made by the flashing process, important strength properties thereof correlate generally with their drainage factor. For a fiber pulp having the same classified fiber length, various strength properties of paper made therefrom increase with the drainage factor of such fibers.

Another indication of improvement of the fibers made in accordance with this invention is their slenderness relative to fibers prepared by typical conventional technique. It is desirable to produce fibers which are relatively thin (or of low coarseness) as these fibers will impart higher opacity, density and better formation to the paper prepared therefrom.

Generally, the set of operating parameters of the present invention will result in improved fiber properties such as thinness and drainage factor compared with fibers prepared using typical conventional parameters. Because other process variables in addition to the specific parameters of this invention also influence fiber properties (e.g., flash nozzle size and configuration, polymer type and molecular weight, solvent, flashing aids, dispersants, etc.), such resulting fiber comparisons are appropriately made with the other process variables held constant.

For the same reason, i.e., the influence of other process conditions besides the parameters specific to this invention on the resulting fiber properties, no absolute fiber property values can be assigned to the fibers which may be prepared by the process of this invention. However, with appropriate selection of all parameters, it has been found that pulps can be produced in accordance with this invention which have a drainage factor in excess of l and as high as 50 to 100 seconds per gram and which have an average coarseness below 15 decigrex (as measured by TAPPI Text 234 SU 67) and frequently between 1 and I decigrex (m/lOO m). For papermaking use, a drainage factor between 2 and may be the preferred range as an appropriate balance between increased strength and ease of water removal from the fibers. Higher drainage factors can be obtained and may be useful where the enhanced strength is more important than rapid water removal.

In the accompanying drawing, the FIG. 1 represents a schematic representation of apparatus suitable for use in carrying out the process of this invention, and FIG. 2 is a detailed representation of the flash nozzle schematically shown in FIG. 1.

In FIG. 1, l is a steam-jacketed vessel, provided with an agitator In, which may be charged with solvent, polymer (or polymer solution) and, if desired, flashing aids such as water. Conduit 2 is provided at the bottom of vessel 1 in communication with flash nozzle 3 through shut-off valve 4 (preferably a ball valve). As seen in FIG. 2 flash nozzle 3 has a substantially smaller diameter than conduit 2. After the mixture is heated to the desired temperature and agitated, if necessary, to dissolve the polymer and/or disperse the flashing aids, valve 4 is opened and the mixture thus formed is forwarded from vessel 1 through flashing nozzle 3 under autogenous pressure of the heated mixture. As the mixture discharges from vessel 1, nitrogen or other inert gas may be introduced into the head space thereof through line 5 in order to maintain the pressure in the vessel at autogenous or higher pressure. The mixture is flashed through nozzle 3 into the flash zone which is comprised of a vapor separation vessel such as cyclone 6, and connecting conduit 7. Conduit 7 is a pipe having an internal cross-sectional area larger than that of flash nozzle 3 and of sufficient internal diameter to permit rapid, unrestricted passage of the flashed noodle to eyclone 6 and preferably has an internal cross-sectional area many times larger than that of the nozzle 3. Optionally, low pressure steam may be introduced into conduit 7 through line 8 located shortly beyond flash nozzle 3 to aid in the vaporization of solvent from the precipitated noodle.

Vaporized solvent and water leaving cyclone 6 pass through line 21 to a solvent recovery unit (not shown).

The flashed fibrous noodle entering cyclone 6 through conduit 7, together with the unvaporized portion of water or other flashing aid present, passed downward through conduit 9 into primary disc refiner 10 where it is lightly shredded. Conduit 11 is provided for introduction of dilution water at an appropriate temperature to form a mixture or slurry with the fibrous material having an appropriate consistency for disc refining.

The lightly shredded material leaving the primary refiner 10 passes through conduit 12 to first receiving tank 13. Nitrogen is introduced into tank 13 through line 14 which sweeps out solvent vapors which are removed via suitable means not shown.

The water slurry of refined fibers is cooled, either by letting the slurry sit in tank 13 for an appropriate time or by other means. The slurry may be withdrawn from receiving tank 13 through line 15 utilizing pump 16, for secondary refining in disc refiner 17. The fibrous slurry leaving secondary refiner 17 passes through conduit 18 to second receiving tank 19 where it is collected prior to further processing for shipment or for papermaking use. If desired, the fibrous slurry leaving refiner 17 may be recycled through line 20 for one or more additional passes through refiner 17.

The fibers after refining may be diluted to a suitable consistency and made into synthetic paper webs, either alone or blended with normal cellulose papermaking fibers. Alternatively, the fibers can be dewatered, pressed into bales, stored and shipped to the ultimate user. The secondary refining previously referred to can be carried out by the ultimate user The illustrated apparatus may be operated on a batch basis as described or on a continuous basis by continuously feeding vessel 1 with polymer solution and any flashing aids desired at flow rates to maintain the appropriate mixture in the vessel for flashing, while heating the vessel to maintain the mixture at the appropriate flash temperature. In order to insure unifrom dispersion of flashing aids, it may be desirable to place an in-line mixing device in line 2 between vessel 1 and flash nozzle 3.

Optionally, instead of using a stirred heated vessel such as vessel 1, it is also possible to prepare the mixture on a continuous basis by blending a polymer solution with any flashing aids desired (as by adding super heated water) continuously in an in-line mixing device just prior to flashing through a nozzle. It is also possible to utilize an in-line mixer.

The following examples will illustrate the invention:

EXAMPLE I The apparatus employed in this example is that illustrated in the drawing and previously described. The dissolution vessel 1 was a two-gallon stirred autoclave equipped with five 4-inch impellers on a single shaft rotated at 1,000 rpm.

The vessel was charged with ().32 Kg of polyethylene (Mitsui 22()()P) having an intrinsic viscosity )1;) of L4 dl/gram and 4.0 liters of n-hexane. The vessel was sealed and the contents raised to a temperature of 143 C. For liters of water containing 2% by weight of 88% hydrolyzed polyvinyl alchol (Moviol 30-88 made by Farbwerke Hoechst AG) based on the polyethylene at a temperature of 143 C. was then pumped into the vessel under agitation. The vessel was pressurized with nitrogen to maintain a pressure of 160 psig. The heated contents were then flashed through a nozzle 3 having a diameter of 0.07 inch 1.75 mm) and a length of 1 Vs inches (28.125 mm) into receiving vessel 6. Low pressure steam psig) was introduced into conduit 7 via line 8 at a point 3 inches downstream of nozzle 3. Dilution water at a temperature of 93 C. was introduced into receiving vessel 6 to provide a slurry having a consistency of about 1.0% by weight and a temperature between 85 C. and 93 C. The aqueous fibrous slurry was then passed through a primary refining zone consisting of a Sprout Waldron single disc refiner having 12-inch plates (Pattern C29-78B) and a plate clearance of 0.150 inch. The moving disc was operated at 1,750 rpm (peripheral velocity of 5,495 feet per minute). The refiner had been preheated with dilution water and steam before refiner start-up and setting of plate clearances.

The pulp slurry was then subjected to secondary refining by passing it at a temperature of 76-80 C. once through the same disic refiner with the plate spacing set at 0.025 inch and once through at 0.010 inch, then twice through at 0.002 inch.

The resulting pulp had a drainage factor of 4.6 seconds/gram and a fiber fraction on the plus 35-mesh screen of 38.4% l-Iandsheets were made from the pulp in accordance with TAPPI Standard Test T-205M-58 with modified wet pressing (400 psig). The handsheets were tested and had the following properties:

Basis weight. g/m" 61.2

Breaking length, Km 0.56 Tensile Energy Absorption, Kg-cm/cm" 0.009 Scott Internal Bond, Kg-cm/cm X 10" 59 Scattering coefficient 1834 In the foregoing example and subsequent examples, the sheet density, basis weight and caliper were determined by TAPPI Standard T-220, tear strength by TAPPI Standard T-4l4, opacity by TAPPI Standard T-425 m-60, fiber fractionation by TAPPI Standard T- 233-SU64, coarseness by TAPPI Standard T-234 SU67, and tensile strength, stretch, tensile energy absorption and breaking length were determined by TAPPI Standard T-494. Scattering coefficient was determined in accordance with the procedure described in the book by Deane B. Judd, Color and Business Science Industry," pp 314-329 (1961) John Wiley and Sons.

The Scott lnternal Bond was determined by employing the Scott instrument under the standard procedure.

The drainage factor in this and following examples was determined substantially in accordance with TAPPI Test T221 05-63 with a slight modification in the method of calculation. Briefly, approximately ten grams of a fiber sample is weighed and dispersed in water. The slurry is then added to the standard sheet mold and water added to the mark. The slurry is stirred by four up-and-down strokes of the standard stirrer, which is then removed. The water temperature in the mold is measured and the drainage valve opened. The time between the opening of the valve and the first sound of suction is noted. The procedure is repeated with water only (no fiber) in the sheet mold and the temperature and drainage time noted. The drainage factor in seconds per gram is then calculated as follows:

Comparative Example 1 In order to show the improvement in fiber properties obtained by subjecting a fibrous noodle to a light defibering action at an elevated temperature rather than a rigorous fibrillating action, Example I was repeated, except that the primary refining was accomplished by passing the fibrous noodle through the refiner with the plates set only 0.005 inch apart. The pulp was then passed once through a secondary refining zone as in Example 1. The resulting pulp had a fiber fraction on the 20 plus 35-mesh screen of 36.4% and a drainage factor of only 1.4 seconds/gram compared to 4.6 seconds/gram for the pulp of Example 1. The lower drainage factor is indicative of a fiber which will form a much weaker sheet, as shown from the handsheet data set forth in the following table. The handsheets were made as in Example 1:

Property 1009? Breaking length, Km 0.22 Tensile Energy Absorption,

Kg-cm/cm 0.001 Scott Internal Bond,

Kg-cm/cm X 10" 31 Scattering Coefficient 1529 EXAMPLE 2 Property 100% *50/50 Blend Breaking Length, Km 0.77 2.2 Tensile Energy Absorption. I Kgem/cm 0.028 0.023 Scott Internal Bond Kg-cm/cm" X 74 140 Scattering Coefficient *The 50/50 Blend refers to a blend of the mlyethylene fibers with bleached alder krutt pulp having a Canadian Standard Freeness of 150 cc.

EXAMPLE 3 EXAMPLE 4 The same general procedure as employed in Example 1 was repeated employing a polyvinyl alcohol made by Farbwerke Hoechst AG (Moviol 75-88) that is 88% bydrolyzed and has a viscosity (4% by weight aqueous solution at 20 C.) of 20-25 centipoises. Runs 1 and 2 were subjected to primary refining in the refiner described in Example 1 at a plate clearance of 0.150 inch. Run 1 was subjected to one pass of secondary refining at a plate clearance of 0.005 inch and two passes of secondary refining at a plate clearance of 0.002 inch at a temperature of 77-80 C. Run 2 was subjected to one pass of secondary refining at a plate clearance of 0.005 inch and three passes of secondary refining at a plate clearance of 0.002 inch at a temperature of 3341 C. Run 3, a comparison, was subjected to one pass of primary refining at a plate clearance of 0.005 inch. Handsheets (both 100% and 50/50 blends) were made and tested as in Example 1. The properties of the fibers and handsheets were as follows:

plate clearance of 0.002 inch at a temperature of 76-77 C. Run 2 was subjected to one pass of second ary refining at a plate clearance of 0.005 and three passes at a plate clearance of 0.002 inch at a temperature of 3039 C. Run 3. a comparison, was subjected to primary refining at a plate clearance of 0.005 inch and two passes of secondary refining at a plate clearance of 0.002 inch at a temperature of 7582 C. Handsheets (both 100% and 50/50 blends) were made as in Example 1 and tested. The properties of the fibers and handsheets were as follows:

The same general procedure as employed in Example 1 was repeated employing a polyvinyl alcohol made by Farbwerke Hoechst AG (Moviol 75-98) that is 98% hydrolyzed and has a viscosity (4% by weight aqueous solution at 20 C.) of 25-30 centipoises. Runs l and 2 were subjected to primary refining in the refiner described in Example 1 at a plate clearance of 0.150 inch. Run 1 was subjected to one pass of secondary refining at a plate clearance of 0.005 inch and two passes Comparison Property Run l Run 2 Run 3 Drainage Factor. sec/g. 3.2 5.2 1.5

"/1 on 20+ 35-mesh screen 31.1 40.2 32

1009 Handshccts Basis Weight. g/m 60.4 59.4 592 Breaking l.ength Km 0.60 0.86 032 Tensile Energy Absorption.

Kg-cm/cm 0.012 0.026 0.002

Scott Internal Bond.

Scattering Coefficient 1267 1276 1301 50/50 Handshects Basis Weight. gjm 58.2 61.8 60.0

Breaking Length. Km 2.67 2.84 2.82

Tensile Energy Absorption Kg-em/cm" 0.037 0.05 004 Scott Internal Bond.

Kg-cm/cm X 10 158 164 174 704 703 707 Scattering Cocfiiciem EXAMPLE 7 Example 6 was repeated except that 5% by weight polyvinyl alcohol (based on the weight of the polyethylene) was employed in the dissolution vessel rather than 2%. After one pass of primary refining at a plate clearance of 0.250 inch and one pass of secondary refining at a clearance of 0.005 inch the resulting pulp had a drainage factor of 15.7 seconds/gram. This example illustrates that higher percentages of the'aqueous disas in Example 1. The properties of the fibers and handpersing agent polyvinyl alcohol may be employed, if desheets were as follows: sired.

Comparison Property Run 1 Run 2 Run 3 Drainage Factor, sec/g. 2.3 2.8 1.8

I7! on -mesh screen 38.0 46.8 30.8

10071 Handsheets Basis Weight. glm 58.9 62.5 59.5

Breaking Length, Km 0.55 0.77 0.35

Tensile Energy Absorption,

Kg-cm/cm 0.007 0.012 0.004

Scott Internal Bond.

Kg-em/cm X 10"" 65 121 42 Scattering Coefficient 1266 1 1 1045 50/50 Handsheets Basis Weight, g/m 59.4 58.9 59.1

Breaking Length. Km .81 2.94 2.43

Tensile Energy Absorption,

kg-emlcm 0.045 0.05 0.04

Scott Internal Bond.

Kg-cm/cm" X 10 170 199 165 Scattering Coefficient 667 658 615 EXAMPLE 6 I claim:

'inch and run 2 was subjected to primary refining with the plate clearance set at 0.250 inch. Run 3, a comparison. was subjected to primary refining at a plate clearance of 0.005 inch. All runs were subjected to one pass of secondary refining with the plate clearance set at 0.005 inch and at a temperature of about C. The resulting pulp had the following properties:

Comparison Property Run 1 Run 2 Run 3 Drainage Factor, sec/g. 6.4 87.3 0.57 Surface Area, m /g l 1.3

1 gas adsorption) '/1 on 20 35-mesh screen 20 Handsheets (both 100% and /50 blends) were made from the pulp of run 1 and tested as in Example 1. The properties were as follows:

Property 100% 50/50 Blend Basis Weight. g/m" 59.1 5&2 Tensile, Kg/l5 mm 1.19 2.51 Breaking Length. Km 1.345 2.874 Tensile Energy Absorption.

Kg-cm/cni" 0.056 0034 Scott lntcrnal Bond.

Kg-em/cm" X 10 1 I4 14) Opacity. 15 (TAPPl corrected) 97.8 95.1)

1. In a method of producing a pulp of polymeric fibers wherein a solution or dispersion of a polymer in a a temperature from above the boiling point of the sol-' vent to 100C in order to vaporize residual solvent, passing the fibrous product at a temperature in such range through a primary refining zone under conditions such that only a light defibering action is imparted to the fibrous product. and subjecting the fibrous product to secondary refining to produce said pulp.

2. The process of claim 1 wherein the temperature of the fibrous product being fed to the primary refining zone is maintained between about C. and C.

3. The process of claim 2 wherein the fibrous product is diluted with water to a consistency between about 1% and 10% by weight prior to primary refining.

4. The process of claim 1 wherein the primary refining is carried out by passing the fibrous material through a double disc refiner with the spacing between the plates set at between about 0.1 inch and 0.25 inch.

5. The process of claim 4 wherein the discs have a relative peripheral velocity between about 4,000 and 8,000 feet per minute.

6. The process of claim 1 wherein the polymer is an at least partially crystalline polyolefin.

7. The process of claim 1 wherein the polyolefin is polyethylene.

8. The process of claim 1 wherein a dispersion of the polymer, solvent and water at a temperature above the melt dissolution temperature of the polymer is flashed through the nozzle.

9. The process of claim 8 wherein the dispersion additionally contains polyvinyl alcohol.

10. The process of claim 1 wherein the fibrous product is cooled to a temperature between about 15 C. and 50 C. prior to the secondary refining.

11. The process of claim 1 wherein the secondary refining is carried out under conditions to impart a vigorous fibrillating action to the fibrous product to thereby produce a pulp having a drainage factor greater than 1.0 second/gram.

12. The process of claim 10 wherein the secondary refining is carried out by passing the cooled fibrous product at least once through a disc refiner whose plates are spaced apart a distance less than about 0.1 inch.

13. In a method of producing a pulp of polyolefin fibers wherein a solution or dispersion of a polyolefin in a solvent at an elevated temperature and pressure is flashed through a nozzle into a zone of reduced pressure to form a fibrous product, the improvement comprising diluting the fibrous product with water to a consistency between about 1% and 10% by weight while maintaining the fibrous product above the boiling point of said solvent at a temperature between about C. and 100 C., in order to vaporize residual solvent passing the fibrous product at said temperature through a primary refining zone comprised of a disc refiner adjusted to impart only a light defibering action to the fibrous product, and passing the fibrous product at least once through a secondary refining zone comprised of a disc refiner adjusted to impart a vigorous fibrillating action to the fibrous material to thereby produce a pulp having a drainage factor greater than 1.0 second/ gram.

14. The process of claim 13 wherein the refiner plates of the double disc refiner of said primary refining zone are spaced apart a distance between about 0. l and 0.25 inch.

15. The process of claim 14 wherein the discs have a relative peripheral velocity between about 4,000 and 8,000 feet per minute.

16. The process of claim 1 wherein the size of the fibrous product after the primary refining step is such that at least of the product is retained on 20 plus 35 mesh screens. 

1. IN A METHOD OF PRODUCING A PULP OF POLYMERIC FIBERS WHEREIN A SOLUTION OR DISPERSION OF A POLYMER IN A SOLVENT AT AN ELEVATED TEMPERATURE AND PRESSURE IS FLASHED THROUGH A NOZZLE INTO A ZONE OF REDUCED PRESSURE TO FORM A FIBROUS PRODUCT UNDER CONDITIONS SUCH THAT SUBSTANTIALLY ALL THE SOLVENT IS VAPORIZED, THE IMPROVEMENT COMPRISING ADDING DILUTION WATER TO THE FIBROUS PRODUCT WHILE MAINTAINING THE FIBROUS PRODUCT AT A TEMPERATURE FROM ABOVE THE BOILING POINT OF THE SOLVENT TO 100*C N ORDER TO VAPORIZE REDISUAL SOLVENT, PASSING THE FIBROUS PRODUCT AT A TEMPERATURE IN SUCH RANGE THROUGH A PRIMARY REFINING ZONE UNDER CONDITIONS SUCH ONLY A LIGHT DEFIBERING ACTION IS IMPARTED TO THE FIBROUS PRODUCT, AND SUBJECTING THE FIBROUS PRODUCT TO SECONDARY REFINING TO PRODUCE SAID PULP.
 2. The process of claim 1 wherein the temperature of the fibrous product being fed to the primary refining zone is maintained between about 70* C. and 100* C.
 3. The process of claim 2 wherein the fibrous product is diluted with water to a consistency between about 1% and 10% by weight prior to primary refining.
 4. The process of claim 1 wherein the primary refining is carried out by passing the fibrous material through a double disc refiner with the spacing between the plates set at between about 0.1 inch and 0.25 inch.
 5. The process of claim 4 wherein the discs have a relative peripheral velocity between about 4,000 and 8,000 feet per minute.
 6. The process of claim 1 wherein the polymer is an at least partially crystalline polyolefin.
 7. The process of claim 1 wherein the polyolefin is polyethylene.
 8. The process of claim 1 wherein a dispersion of the polymer, solvent and water at a temperature above the melt dissolution temperature of the polymer is flashed through the nozzle.
 9. The process of claim 8 wherein the dispersion additionally contains polyvinyl alcohol.
 10. The process of claim 1 wherein the fibrous product is cooled to a temperature between about 15* C. and 50* C. prior to the secondary refining.
 11. The process of claim 1 wherein the secondary refining is carried out under conditions to impart a vigorous fibrillating action to the fibrous product to thereby produce a pulp having a drainage factor greater than 1.0 second/gram.
 12. The process of claim 10 wherein the secondary refining is carried out by passing the cooled fibrous product at least once through a disc refiner whose plates are spaced apart a distance less than about 0.1 inch.
 13. In a method of producing a pulp of polyolefin fibers wherein a solution or dispersion of a polyolefin in a solvent at an elevated temperature and pressure is flashed through a nozzle into a zone of reduced pressure to form a fibrous product, the improvement comprising diluting the fibrous product with water to a consistency between about 1% and 10% by weight while maintaining the fibrous product above the boiling point of said solvent at a temperature between about 70* C. and 100* C., in order to vaporize residual solvent passing the fibrous product at said temperature through a primary refining zone comprised of a disc refiner adjusted to impart only a light defibering action to the fibrous product, and passing the fibrous product at least once through a secondary refining zone comprised of a disc refiner adjusted to impart a vigorous fibrillating action to the fibrous material to thereby produce a pulp having a drainage factor greater than 1.0 second/gram.
 14. The process of claim 13 wherein the refiner plates of the double disc refiner of said primary refining zone are spaced apart a distance between about 0.1 and 0.25 inch.
 15. The process of claim 14 wherein the discs have a relative peripheral velocity between about 4,000 and 8,000 feet per minute.
 16. The process of claim 1 wherein the size of the fibrous product after the primary refining step is such that at least 90% of the product is retained on 20 plus 35 mesh screens. 